Fix missing prototype warning for xf86_find_platform_device_by_devnum()

../hw/kdrive/src/kdrive.c:999:1: warning: no previous prototype for ‘xf86_find_platform_device_by_devnum’ [-Wmissing-prototypes]

Place the same guards around this stub as are around including the
hotplug.h header which declares the prototype.
3 jobs for warning-fixes-2 in 6 minutes and 50 seconds (queued for 2 seconds)